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Acid Mine Drainage

Acid Mine Drainage. Mining & the Environment. Mine overburden & waste soils (mine tailings) are waste products generated by the mining industry.

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Acid Mine Drainage

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  1. Acid Mine Drainage

  2. Mining & the Environment • Mine overburden & waste soils (mine tailings) are waste products generated by the mining industry. • When these tailings are exposed to the atmosphere, precipitation and ground or surface water, they can react with oxygen & water to generate products which affect the pH & heavy metal composition of soils & streams

  3. Mine Tailings

  4. Acid Mine Drainage • When mineral deposits containing sulfides are mined, they have the potential to produce acid mine drainage. • Coal, copper, gold, silver, zinc, lead & uranium • AMD is caused by the physical & chemical weathering of the common mineral pyrite (FeS2)

  5. Pyrite • Physical weathering of pyrite is necessary to reduce the grain size of the mineral. • Miners often accelerated this process by grinding up ores and dumping the overburden in the mine tailings piles • When exposed to water & oxygen, pyrite forms sulfuric acid.

  6. Oxidation of Pyrite 4FeS2(s) + 14O2(g) + 4H2O(l) 4Fe 2+(aq) + 8SO42-(aq) + 8H+ • The ferrous & hydrogen ions are released into the waters that runoff from mine drainage tunnels or tailings piles. • The ferrous ions are oxidized to form ferric ions 4Fe 2+(aq) + O2(g) + 4H+(aq) 4Fe3+(aq) + 2H2O(l)

  7. Oxidation of Pyrite • The ferric ion hydrolyzes win water to form an insoluble yellow-orange precipitate called “yellow boy”. 4Fe3+(aq) + 12H2O(l) 4Fe(OH)3(s) + 12 H+(aq)

  8. AMD in the High Andes, Peru

  9. AMD in Colorado

  10. “Yellow boy” precipitation smothers aquatic plants and animals

  11. 4FeS2(s) + 14O2(g) + 4H2O(l) 4Fe 2+(aq) + 8SO42-(aq) + 8H+ 4Fe 2+(aq) + O2(g) + 4H+(aq) 4Fe3+(aq) + 2H2O(l) 4Fe3+(aq) + 12H2O(l) 4Fe(OH)3(s) + 12 H+(aq) 4FeS2(s) + 15O2(g) + 14H2O(l) 4Fe(OH)3(s) + 8SO42-(aq) +16H+ smothers organisms living on the stream bottom

  12. Microbial Influences • Abiotic oxidation of pyrite is slow. • The bacterial microbe Thiobacillus ferrooxidans catalyzes the oxidation of FeS2 to ferric ions and hydrogen ions

  13. Microbial Influences • The pH of AMD can less than 3. • Other heavy metal ions (zinc, copper, lead, arsenic and manganese) are also soluble in acidic solution & are mobilized • Streams are often devoid of life for miles downstream of an AMD source

  14. T. ferrooxidans • Acidophilic • capable of surviving at low pH’s • Autotrophic • obtains its carbon by fixing atmospheric CO2 Viewed by electron microscope magnified 30,000 times

  15. T. ferrooxidans • Obtains its energy by the oxidation of either iron or sulfur Fe 2+ + 0.25 O2 + H+ Fe 3+ + 0.5 H2O H2S + 2O2 SO4 2- + 2H+ So + H2O + 1.5 O2 SO4 2- + 2H+ S2O3 2- + H2O + 2O2 2SO4 2- + 2H+

  16. T. ferrooxidans • T. ferrooxidans is generally assumed to be obligately aerobic, but under anaerobic conditions, it can be grown on elemental sulfur using ferric iron as an electron acceptor. S + 6Fe3+ + 4H2O H2SO4 + 6Fe 2+ + 6H+ G=-314 KJ/mol

  17. T. ferrooxidans • Important in bioleaching processes where anaerobic conditions exist • Can also obtain energy from oxidizing Cu+, Se2+, & from oxidation of Sb, U & Mo compounds Red-orange color due to production of Fe(III) as T. ferrooxidans oxidizes Fe(II)

  18. T. ferrooxidans • Experiments show that T. ferrooxidans accelerates extraction of copper from ores

  19. Coal Mining and AMD Upper Conemaugh River Basin, PA

  20. A Little History • Nature bestowed Cambria & Somerset Counties, PA a mixed blessing with an abundance of coal & a topography which made it easy to extract • Five minable seams of coal provided the energy needed for the Industrial Revolution which made Johnstown one of the largest iron & steel production centers in the world

  21. A Little History • The Cambria Iron Company (Andrew Carnegie’s first still mill) was located in Johnstown • It later grew into the largest integrated Steel Mill in the world (stretched 14 mi along the Conemaugh & Little Conemaugh Rivers • Steel mills used large amount of coal to make coke (fuel for the clast furnaces)

  22. Types of Coal Mines • Drift or Slope Mines • driven into valley walls near level of coal • drain excess water encountered by gravity flow out the entry • Shaft Mines • pumps used to remove water • boreholes drilled to relieve water pressure

  23. Types of Coal Mines • Surface Mines • uses draglines which can remove up to a depth of 200 ft in a single pass • miners left the overburden rock where it acid and metals into streams to add to the discharges from the abandoned deep mines

  24. Water Flows • Underground mines may produce thousand gallon per minute flows • Strip mines produce less flow

  25. Mine Drainage Wasteland • Iron mound precipitated from water discharging from a 300’ deep borehole. • Precipitate (up to 9 ft deep) has killed trees

  26. Open Mine Entry • Water discharging from drift mine. • Discharges from these types of mines • 200-800 gpm • pH range 2.7-3.2 • Metal concentrations: • 58mg/l Fe • 20.9 mg/l Mn • 55.4 mg/l Al

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